Impossible Thief: He goes from a lowly thief in the prisons of Sounis at the beginning of the series to the sovereign of ''all three countries by the end of the series; Eddis, Sounis and Attolia. Though to be fair, he was in prison by his own choice and only pretending to be "lowly".
Jerk with a Heart of Gold: He enjoys provoking the hell out of people, is rude and sarcastic, and generally infuriates everyone he meets, but he's still a good friend who's trying to help the peninsular countries the best he can.
Meaningful Name: Eugenides is also the name of the god of thieves, and as he's the Queen's Thief, it's his name as well as his title.
No Hero to His Valet: As much as his childhood friend and cousin Eddis respects him, she still says explicitly that she would sooner strangle him than marry him. She also tells the woman he's in love with how much of a liar he is.
Obfuscating Stupidity: In the first book. Funny thing is, he's the narrator, using the first-person perspective. Somehow he manages to fool the reader without leaving out anything that would be noticed. This also comes into play in the third book (although this time the reader is in on it), wherein the court in Attolia are lead to think he's an ineffectual Upper-Class Twit.
Pals with Jesus: Eugenides is noted for not only talking to his gods, but for having them talk back. Of course, they tell him things like "STOP WHINING" and "GO TO SLEEP", which he finds to be rather anticlimatic.
Attolia: "I have trusted you, and no, that does not mean that I have not had you watched and that I do not have spies that watch my spies, and spies even that watch those."
Broken Bird: She was pawned off on an asshole husband, and after she took control of her country, she had no choice except to be totally ruthless to maintain her power thanks to the misogyny on top of the fact that her barons are untrustworthy power-grubbers.
Berserk Button: The honor of the Queen's guard, his own honor, honor in general.
The Chew Toy: Most of the third book is about him getting punished, ostracized, provoked, humiliated, being called out in the middle of the night because the king has gone nuts...
Did I Say That Out Loud: Costis' response to Eugneides saying the way he can get out of serving him is if both he and Attolia were killed and he instead served their heir, an option Eugenides says is unlikely. To which Costis asks if he means both of them dying or them making a heir. It's only after the words leave his mouth that he realizes what a huge mistake he made in saying them...
Honor Before Reason: Seems to be a theme with members of the Guard, but even they think Costis takes it too far.
Knight in Shining Armor: He sees himself as one to Attolia, mentally referring to her as "my Queen" rather than the more proper "Your Majesty".
Badass Bookworm: He's probably the most scholarly character in the books, but he's a skilled fighter and an incredibly capable royal servant.
Be Careful What You Wish For: In Queen of Attolia, he tries to return Eugenides' spirits by saying there are still things he can do. Eugenides takes him at his word and steals the magus by framing him for the destruction of Sounis' navy. The magus didn't mean he should do that!
Beware the Nice Ones: He tries to win over the barons with an idealistic speech about how they need to ally with Eddis and Attolia. It doesn't work. So he shoots the chief conspirator against him very dead and asks for a new vote. They confirm him as King unanimously.
Break the Cutie: During A Conspiracy of Kings, when he is kidnapped, becomes a slave, and is generally thrust neck-deep into the cutthroat politics of his world, where everyone is trying to manipulate him.
The Bus Came Back: He doesn't appear at all in the middle two books, but he's the protagonist of the fourth.
Comes Great Responsibility: During the fourth book, he realizes that he's done his family, servants, and country a huge disservice by not putting any effort into being a plausible, intimidating heir to his uncle.
Heroic Self-Deprecation: He thinks of himself as a spineless weakling unworthy to succeed his uncle. At one point he says that he has a gift, which is knowing when he is an ass.
Made a Slave: He's hidden by his kidnappers this way in Conspiracy of Kings, along with some beating to make his face unrecognizable. He actually likes it better than being the heir.
Obfuscating Stupidity: In A Conspiracy of Kings. Everyone thinks he's an idiot and his emotions show on his face. So he lets everyone think he's a gormless, naive dolt while he plans to show them all that he is 1. smart and 2. definitely ruthless enough to succeed his uncle.